How Do Activated-Carbon Filters Adsorb Pollutants So Effectively?

1. Honeycomb-Like Porous Network

Activated carbon is made from wood, coal, fruit shells, and other raw materials through high-temperature activation. The process creates a large number of interconnected micropores, mesopores, and macropores. Pore diameters generally range from 1 to 1,000 nm, while each gram of activated carbon can provide a specific surface area of 500-2,000 m², approximately the area of a soccer field. This sponge-like structure provides an enormous number of surface adsorption sites and makes it easier for pollutant molecules to contact the carbon.

2. Pore-Size Matching

Micropores, below 2 nm: These account for the largest share and mainly adsorb small molecules such as formaldehyde, benzene, and odor molecules. They are the principal source of activated carbon's adsorption capacity.

Mesopores, 2-50 nm: These act as transport channels, guiding pollutants in gases or liquids into the micropores. They can also adsorb medium-sized molecules such as pigments and oils.

Macropores, above 50 nm: These mainly provide bulk transport pathways that help pollutants diffuse quickly into the internal pore network.

Physical Adsorption: Molecular Capture by Van der Waals Forces

Physical adsorption relies on van der Waals forces, which are weak intermolecular forces between the activated-carbon surface and pollutant molecules. Because these forces exist between all molecules, activated carbon can adsorb most organic compounds, odor molecules, pigments, and similar substances.

This is a reversible process. When the pollutant concentration in the surrounding environment falls, some molecules may desorb, particularly at high temperatures or low ambient concentrations. Physical adsorption is nonselective and is more effective for substances with higher molecular weights and boiling points, including benzene-series compounds and TVOCs, because they experience stronger van der Waals forces.

Chemical Adsorption: Targeted Capture Through Chemical Bonds

The surface of activated carbon contains functional groups such as hydroxyl, or -OH, carboxyl, or -COOH, and aldehyde groups. These can react chemically with specific pollutants and form chemical bonds.

Chemical adsorption is irreversible: after the reaction, pollutants are difficult to desorb, making it a form of permanent adsorption. It is also selective and works only on substances capable of reacting with the surface functional groups.

In everyday applications, activated carbon works best on nonpolar or weakly polar molecules such as organic compounds, odors, and pigments, but is less effective on strongly polar inorganic substances such as ammonia. Higher temperatures reduce physical adsorption but may promote some chemical adsorption. At high humidity, water molecules occupy the pores and reduce adsorption of other pollutants. Activated carbon made from different raw materials and activation processes has different pore structures and surface functional groups, so it should be selected for the application. Coconut-shell activated carbon, for example, is commonly chosen for air purification, while coal-based activated carbon is commonly used for water treatment.